Urethane materials obtained by a reaction between an isocyanate compound and a hydroxyl group-containing compound such as a hydroxyl group are used in various fields.
For example, polyurethane resins produced by allowing a polyisocyanate compound to react with a polyol such as a diol in the presence of a catalyst and, if necessary, additives such as a foaming agent, a surfactant, and a cross-linking agent are widely used in applications such as automobiles, architectures, household electric appliances, heavy duty coatings, plastic coatings, and adhesives because curing reactions of such polyurethane resins proceed even at room temperature, resins having a cross-linked structure can be formed, and such polyurethane resins are excellent in terms of flexibility and adhesiveness with base materials.
Furthermore, urethane (meth)acrylates produced by allowing a compound having a hydroxyl group and a radical polymerizable group, such as hydroxylethyl(meth)acrylate, to react with a polyisocyanate in the presence of a catalyst or by allowing a compound having an isocyanate group and a radical polymerizable group, such as (meth)acryloyl isocyanate, to react with a polyol in the presence of a catalyst are widely used as active-energy-ray-curable resins, specifically, as protective film materials and adhesives of various base materials such as glass, ceramics, metals, paper, and wood because such urethane (meth)acrylates have characteristics such as sufficient strength and flexibility after curing, and various resins can be designed depending on the backbone of the polyol.
Furthermore, two-liquid type urethane-forming compositions that utilize a urethane-forming reaction are used in polyurethane paintings, adhesives, formed articles, sealants, production of hard or soft foams, and elastomers.
Isocyanate compounds used in a urethane-forming reaction include aromatic isocyanates typified by tolylene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI); and aliphatic isocyanates typified by dicyclohexylmethane diisocyanate (HMDI) and isophorone diisocyanate (IPDI). Although yellowing caused by light or heat does not readily occur in the latter isocyanate compounds, the rate of reaction with a hydroxyl group-containing compound is significantly lower than that of the former isocyanate compounds, and thus highly active catalysts have been desired. As urethane-forming reaction catalysts used in urethane-forming reactions, metal catalysts are widely used. Organotin catalysts are used because of high activity thereof, and dibutyltin dilaurate (DBTDL) or stannous octoate are mainly used (refer to, for example, Non-Patent Document 1).
However, as for the organotin catalysts, recently, a problem of toxicity of organotin catalysts has been pointed out. In particular, tributyltin contained as an impurity in DBTDL causes a problem of hazardousness to the human body as an environmental hormone. Thus, alternative catalysts have been desired.
It is known that compounds of lead, mercury, or bismuth also accelerate urethane-forming reactions. However, since these heavy metal compounds have high toxicity, the use of these compounds also tends to be suppressed similarly to organotin compounds. It has long been known that compounds of a transition metal such as iron, copper, titanium, zirconium, nickel, cobalt, or manganese, in particular, acetylacetonato complexes of these metals have high urethane-forming activity (refer to, for example, Patent Document 1).
As non-metal catalysts, tertiary amine catalysts are used, but the catalytic activity thereof is low. Therefore, a method in which a tertiary amine catalyst is added to a metal acetylacetonato complex has been proposed (refer to, for example, Patent Document 2 and Patent Document 3). According to the methods described in the documents, the catalytic activity increases, and a curing rate substantially the same as that when an organotin catalyst is used can be achieved. However, these methods have a problem that a metal compound used as the catalyst remains in the resulting product.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 09-031151    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-82052    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-231878    Non-Patent Document 1: “Poriuretan no Kouzou/Bussei to Koukinouka oyobi Ouyoutenkai (Structures/physical properties of polyurethane, improvement in function thereof, and development of application thereof)” authored by Tetsuo Yokoyama, published by Technical Information Institute Co., Ltd., issued in 1998, page 325